Process for producing isoprene



United States Patent 3,211,737 PROCESS FOR PRUDUCING ISOPRENE Emmett H. Bnrk, Jr., Hazel Crest, and Byron W. Turnquest, Chicago, 111., assignors to Sinclair Research, Inc., Wilmington, DeL, a corporation of Delaware No Drawing. Filed Nov. 27, 1961, Ser, No. 155,198 5 Claims. (Cl. 260680) This invention relates to a process for the production of isoprene, more particularly the present invention relates to the production of isoprene by the thermal pyrolysis of 3,3-dirnethyl-u-olefins.

In view of the similarity of cis-polyisoprene, i.e. synthetic natural rubber, to natural rubber, the demand for isopropene is increasing. The availability and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thus, there is a continuing search for new sources of isoprene production.

A great deal of research has been directed recently to the production of isoprene by dehydrogenation of C stocks by processes similar to those employed for the production of butadienes. However, these processes have disadvantages which limit their usefulness. Dehydrogenation of isopentane, which is available in relatively large quantities from normal refinery streams and natural gas or field gas streams, is ordinarily carried out by a one step process such as a proces using a chromium-alumina catalyst. In this process, isomerization occurs during dehydrogenation resulting in the formation of one pound of piperlylene (straight chain C diolefins) for every 2.5 pounds of isoprene in the product.

Furthermore, the quantity of isoamylenes which are available from catalytic cracking or steam cracking of gas oil i quite limited relative to the potential demand for isoprene according to the predicted potential demand for isoprene according to the predicted shortage of 600,000 long tons of natural rubber by 1965. Moreover, none of the special catalytic methods is capable of effecting dehydrogenation to isoprene in a desirably selective manner. In all cases, :an appreciable amount of the hydrocarbon feed and/ or reaction products is consumed in side reactions such as cracking, polymerization and oxidation.

It has also been reported that the thermal cracking of olefins such as Z-methylpentene-Z affords a relatively simple method for isoprene production. In obtaining maximum selective yields under this process, however, it has been found that the degree of cracking is relatively low. Low conversion levels are disadvantageous in that they complicate the separation of the isoprene from the crude pyrolyzate stream.

Emmett H. Burk, Jr. and William D. Hoffman disclose in their application Serial No. 122,082, filed July .6, 1961, which is a continuation-in-part of application Serial No. 94,955, filed January 11, 1961, now abandoned, a process for the production of high selective yields of isoprene by the thermal cracking or pyrolysis of 3,3-dimethyl-a-olefins and that the process can be run at high conversion levels (e.g. or about 75%) without formation of n-pent-anes. Also disclosed in the above application is the thermal cracking of 3,3-dimethyl-a-olefins in the presence of hydrogen halide as catalyst results in high selective yields of isoprene at high conversion levels with large concentrations of isoprene in the C reaction product.

We have now found that the concentration of isoprene in the C fraction resulting from the pyrolysis of 3,3-dimethyl-a-olefins in the presence of HX wherein X is a halogen of 35-53 atomic number, can be increased by the addition of ethylene to the cracking reaction. The importance of increasing the isoprene concentration in the C fraction, however slight, cannot be over emphasized particularly from the economical viewpoint of isoprene recovery. Due to the close boiling points of the C components, isoprene must often be concentrated by expensive procedures such as extractive distillation or solvent extraction. Therefore, any improvement which can be made in isoprene concentration in the crude reaction product is very important. This is particularly true for the thermal cracking of 3,3-dimethyl-u-olefins of over 6 carbon atoms since these substances crack very selectively to 0;, species but give only moderate concentrations of isoprene in the C fraction.

The olefin feedstock of thi invention has the formula:

wherein R is an alkyl radical of '1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e. of 3 to 6 carbons), the branched structure being preferred. Suitable olefin feeds include, for instance, 3,3-dimethylbutene-l; 3,3-dimethylpentene- 1; 3,3-dimethylheXene-1; 3,3,4trimethylpentene-1; 3,3-dimethyl-5,S-dimethylhexene-1; etc.

3,3-dimethyl-ot-olefins can be prepared by any method known to the art. For instance, 3,3-dimethyl butene-l can be obtained by a process which comprises dimerization of acetone by aluminum amalgum to form pinacol, rearranging the pinacol to pinacolone by treatment with an acid catalyst, reducing the pinacolone by catalytic methods to the corresponding alcohol, 3,3-dimethylbutanol-2, forming a Xanthate or other appropriate ester of the alcohol and subjecting it to thermopyrolysis to obtain 3,3- dimethylbutene-l. 3,3-dimethybutene-1 has also been obtained by the pyrolysis of the stearic acid ester of 3,3- dimethylbutanol-Z (see Koch and Van Ra-ay, Greenstoff- Chemie, 32, 161-174, 1951). The preferred, and more practical process for the production of 3,3-dimethylbutene- 1 is disclosed in application Serial No. 94,956 to Emmett H. Burk, Jr., and William D. Hoffman, filed January 11, 1961, now abandoned. Briefly the rocess of the application involves dehydrochlorinating 1-ch1oro-3,3-dimethylalkane by contact with a solid inert contact material at a temperature of about 450-600 C. and a residence time of about .01 to 10 seconds.

In accordance with the process of the present invention the a-olefin feed is subjected to thermal cracking general- 1y at a temperature of about 1000" F. to 1600 F. in the presence of at least 1 mol percent of a hydrogen halide, preferably 2 to 15 mol percent, based on the a-olefin feed and in the presence of 0.5 to or more mol percent, preferably 5 to 50 mol percent of ethylene based on the a-olefin feed. The ethylene is present at the start of the reaction or substantially throughout the reaction period, that is, for a reaction time which in the presence of the ethylene is sufiicient to provide an enhanced overall yield of isoprene. The process can be conducted under vacuum or in the presence of an inert gas diluent such as helium, steam, etc. Operating conditions can be adjusted to give an u-olefin partial pressure of up to about 0.5 atmosphere, for instance, .05 to 0.5 atmosphere, and a contact time sufiicient to produce isoprene, for instance about .001 to 1 second. An inert gas, if employed, can be supplied in a molar ratio of at least about 1 mole, for instance, about 1 to 40 or 100 or more moles, preferably 15 to 30 moles per mole of hydrocarbon. Under preferred conditions the feed partial pressure is about 0.1 atmosphere and the contact time is about 0.05 to 0.1 second. A reaction temperature of about 1200 to 1400 F. is preferred. If desired, the inert gas can be employed as the heating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature above that desired for conductnig the cracking operation, generally at least about 25 C. higher than the reaction temperature and not above say about 200 C. of the reaction tempenature. The inert gas is then quickly mingled with the hydrocarbon which is at a temperature below that at which any reactions occur. The temperature to which the inert gas is to be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. The product from the thermal cracking process is quenched to a temperature of about 500 F. or below and fractionated in ordinary fractionating equipment to obtain isoprene of at least about 95% purity.

The hydrogen halide catalyst that may be used in the present invention can be hydrogen iodide or bromide per se or the catalyst can be supplied by the iodide or bromide compounds, which decompose and/or dissociate under the reaction conditions to produce hydrogen halide and whose presence is not otherwise detrimental to the desired reaction. Illustrative of suitable halides are hydrocarbon halides such as methyl iodide or bromide, tertiary butyl iodide or bromide, elemental iodine or bromine, or inorganic iodides and bromides such as hydrogen iodide or bromide.

The following examples will serve to illustrate the present invention.

EXAMPLE I A l'2-inch furnace was equipped with a quartz reactor. A feed consisting of 92.6% 3,3-dimethylbutene-1 and 7.4% isoamylenes to which was added hydrogen iodide and ethylene in the amounts shown in Table I below was introduced into a stream of steam and the mixture was passed through the reaction zone under the conditions shown in Table I below. After leaving the reaction zone, the gas stream was sampled and analyzed by vapor phase chromatography. For comparison a run was also made without ethylene addition. The results are reported in Table I below;

! Feed consists of 92.6% 3,3-dimethylbutene-1 and 7.4% isoamylenes.

We claim:

1. A process for producing isoprene which comprises subjecting 3,3-dimethyl-a-olefin to thermal pyrolysis at a temperature of about 1000 to 1600 F., a 3,3-dimethyla-olefin partial pressure of up to about 0.5 atmosphere and in the presence of at least about one mol percent of hydrogen iodide, and at least about 0.5 mol percent of added ethylene based on said a-olefin, said amount of ethylene being present substantially throughout the reaction and said 3,3-dimethyl-a-olefin being of the formula:

wherein R is an alkyl radical of 1-6 carbon atoms.

2. The process of claim 1 wherein the olefin is 3,3- dimethylbutene-l.

3. The process of claim 1 wherein the etheylene is present in an amount of about 5 to 50 mol percent.

4. The process of claim 2 wherein an inert gas in a molar ratio of at least one mol of inert gas per mol of 3,3-dimethylbutene-1 is employed.

5. A process for producing isoprene which comprises subjecting 3,3-dimethylbutene-1 to thermal pyrolysis at a temperature of about 1200 to 1400 F., a 3,3-dimethylbutene-l partial pressure of up to about 0.5 atmosphere and in the presence of at least about 1 mol percent of hydrogen iodide, at least about 0.5 mol percent of added ethylene based on said a-olefin, said amount of ethylene being present substantially throughout the reaction, and at least about 1 mol of inert gas per mol of 3,3-dimethylbutene-l.

References Cited by the Examiner FOREIGN PATENTS 1,251,127 12/60 France.

OTHER REFERENCES Derwent Belgium Patents Report, published by Derwent Information Service, London, England, vol. 76 B, page A3, June 30, 1961.

ALPHONSO D. SULLIVAN, Primary Examiner. 

1. A PROCESS FOR PRODUCING ISOPRENE WHICH COMPRISES SUBJECTING 3,3-DIEMTHYL-A-OLEFIN TO THERMAL PYROLYSIS AT A TEMPERATURE OF ABOUT 1000 TO 1600*F., A 3,3-DIMETHYLA-OLEFIN PARTIAL PRESSURE OF UP TO ABOUT 0.5 ATMOSHPERE AND IN THE PRESENCE OF AT LEAST ABOUT ONE MOL PERCENT OF HYDROGEN IODIDE, AND AT LEAST ABOUT 0.5 MOL PERCENT OF ADDED EHTYLENE BASED ON SAID A-OLEFIN, SAID AMOUNT OF ETHYLENE BEING PRESENT SUBSTANTIALLY THROUGHOUT THE REACTION AND SAID 3,3-DIMETHYL-A-OLEFIN BEING OF THE FORMULA: 